The present invention relates to a scanning optical system employed, for example, in a color laser beam printer, color laser copier or the like.
Color printers have been developed and widely used for an output device of computers, color copiers or the like. In order to form a color image at a high speed, a color laser printer is widely employed. The color laser beam printer typically includes a plurality of photoconductive drums, and corona chargers, developing units etc. for respective color components, i.e., yellow (Y), magenta (M), cyan (C) and black (B).
In order to form electrostatic latent images consisting of the four color components on the photoconductive drums, respectively, four scanning optical systems, each of which has a polygonal mirror and imaging optical system, may be used. However, such an apparatus provided with four polygonal mirrors and four imaging optical systems for four photoconductive drums costs a lot in manufacturing thereof. Recently, an apparatus making use of a single polygonal mirror for simultaneously deflecting four beams corresponding to the four color components has been developed. The four deflected beams are incident on four imaging optical systems, respectively, thereby the four beams being directed to the four photoconductive drums. Thus, four image components can be formed simultaneously.
In order to simultaneously deflect a plurality of laser beams using a single polygonal mirror, in particular, to make a plurality of beams be incident on the plurality of imaging optical systems arranged separately in the auxiliary scanning direction, respectively, the beams should be inclined at different angles with respect to a plane perpendicular to a rotational axis of the polygonal mirror, or the beams are separated from each other in the auxiliary scanning direction.
If laser beams are inclined with respect to a plane perpendicular to the rotation axis of the polygonal mirror, the scanning lines, which are loci formed by moving beam spots on the surfaces to be scanned, curve in the auxiliary scanning direction. By displacing one of the lenses of the imaging lens, which has a power to converge a beam in the auxiliary scanning direction and is arranged at a position closer to the surface to be scanned than the other lenses, in the auxiliary scanning direction with respect to the beams incident on the lens, the curvature of the scanning lines in the auxiliary scanning direction, or a bow can be suppressed by a certain degree. However, the displacing amount varies depending on the incident angle of the beams with respect to the polygonal mirror. Therefore, such a lens should be formed to have a relatively wide effective area, which makes it very difficult to manufacture the lens. Further, in the above-described optical system, four of such lenses are required, which increases the manufacturing cost of the scanning optical system.
If a plurality of laser beams, which are separately arranged in the auxiliary scanning direction, are perpendicularly incident on the reflection surface of the polygonal mirror, the laser beams can be directed to proceed in parallel with the optical axes of the imaging optical systems, respectively, and therefore, the curvature in the auxiliary scanning direction can be prevented. However, in such a configuration, the size of the polygonal mirror in the auxiliary scanning direction should be increased. Further, due to upsizing of the polygonal mirror, a motor for rotating the polygonal mirror should have sufficient power, which also increases the manufacturing cost of the entire optical system.
The present invention is advantageous in that, a scanning optical system is provided, in which a plurality of beams incident on a single polygonal mirror are inclined with respect to a plane perpendicular to the rotation axis of the polygonal mirror, and the bow of the scanning lines can be well suppressed using inexpensive lenses.
According to the present invention, there is provided a scanning optical system that emits a plurality of beams to a plurality of surfaces to be scanned, respectively. The scanning optical system includes a light source and a polygonal mirror. The plurality of beams incident on the polygonal mirror are inclined with respect to each other in an auxiliary scanning direction. The scanning optical system further includes an imaging optical system that converges the deflected beams on the plurality of surfaces, respectively. The imaging optical system includes a front lens group and a plurality of rear lens groups. All the beams are incident on the front lens group, and then incident on the respective rear lens groups. Each of the plurality of rear lens groups has a shape which is designed in accordance with an angle of a beam incident thereon with respect to an optical axis of the front lens group.
With this configuration, each of the rear lens groups needs not be formed to have a wide effective width in the auxiliary scanning direction, and therefore the manufacturing cost can be suppressed.
Optionally, each of the plurality of rear lens groups has at least one refraction surface which is expressed by a predetermined mathematical expression, each of the plurality of rear lens groups being configured such that a reference axis of the at least one refraction surface is displaced with respect to a center of a shape of the at least one refraction surface so that shifting amounts of positions where the beams incident on the at least one refraction surfaces of the plurality of rear lens groups with respect to the reference axes of the refraction surfaces are different from each other.
Further, the plurality of rear lens groups are configured such that the shifting amounts are greater as the angles of the incident beams with respect to the front lens group are greater, respectively.
Alternatively, each of the plurality of rear lens groups has at least one refraction surface, the at least one refraction surfaces of the plurality of rear lens groups being expressed by different mathematical expressions which are determined in accordance with angles of the incident beams with respect to an optical axis of the front lens group, respectively.
Preferably, the plurality of beams incident on the polygonal mirror intersect with each other at a point in the vicinity of a reflection surface of the polygonal mirror.
In this case, the front lens group may be arranged such that an optical axis thereof is parallel with a plane perpendicular to a rotation axis of the polygonal mirror, the optical axis of the front lens group passing the point at which the plurality of beams intersect with each other, each of refraction surfaces of the front lens group being symmetrical with respect to a main scanning plane which is a plane including the optical axis of the front lens group and perpendicular to the rotation axis of the polygonal mirror.
In a particular case, the plurality of laser beams may include first and second laser beams, which are inclined with respect to the main scanning plane on one side thereof at different angles, and third and fourth laser beams, which are inclined with respect to the main scanning plane on the other side thereof at different angles. Angles at which the first and fourth laser beams incline with respect to the main scanning plane may have the same absolute values, and angles at which the second and third laser beams incline with respect to the main scanning plane may have the same absolute values.
Optionally, two of the plurality of rear lens groups on which the first and fourth laser beams are incident may have the same shapes.
Further, two of the plurality of rear lens groups on which the second and third laser beams are incident may also have the same shapes.
Optionally, each of the plurality of laser beams incident on the polygonal mirror may be inclined with respect to the optical axis of the front lens group when projected on the main scanning plane.
Still optionally, all the plurality of rear lens groups may have different shapes.
Further optionally, each of the plurality of rear lens groups may be an anamorphic optical system having a stronger positive power in the auxiliary scanning direction than in the main scanning direction.